1. Field of the Invention
The present invention relates to a zoom lens which uses a variable refractive power element capable of controlling a refractive power of a lens.
2. Description of the Prior Art
In a prior art imaging optical system which is focused by electronic control such as a zoom lens used in a still camera or a video camera, a focus lens comprising light weight lenses is desirable because of a reguirement of power saving and compactness of a drive motor. In a normal zoom lens, focusing is effected by a front element of the lens unit which is the heaviest one. Japanese Unexamined Patent Publication No. 107347/1978 and 127321/1979 show a method in which the focusing is effected by a small size lens group called a compensator, as shown in FIG. 1A.
In FIG. 1A, F denotes a front lens, V denotes a variator which varies a magnification of an overall system by shifting along an optical axis, C denotes a compensator which is moved along the optical axis to keep the image of the overall system at a constant position, and R denotes a relay lens group. Solid lines shown below V and C show locus of movement of V and C when the lens is zoomed from a wide angle end to a telescope end when an object is at an infinite point, and a broken line shows a similar locus when the object is at a very near point. FIGS. 1B and 1C show similar loci.
As seen from FIGS. 1A to 1C, in such a system, the drive amount for focusing at the telescope end is large. In order to secure a space for the drive amount, it is necessary to increase a spacing between the variator V and the compensator C in FIGS. 1A and 1C, and a spacing between the compensator C and the relay lens R in FIG. 1B. Thus, it is difficult to reduce the overall length of the zoom lens. Further, since an iris is spaced from the front element of the zoom lens, it is difficult to reduce the diameter of the front element. In FIG. 1C, it is principally impossible to focus the lens to a very near object where an image distance to the variator V is less than four times of a focal distance of the variator V.
Since the movement of the compensator is not determined only by the magnification of the front element focusing zoom lens but is determined based on both the distance to the object and the magnification, mechanical control by a cam is not allowed. Accordingly, a relationship among the position of the compensator C, the position of the variator V and the distance to the object is stored, and the position of the variator V and the distance to the object are detected so that the position of the compensator C is determined by the motor control. This system, however, requires a position detector for the variator, and it is difficult attain a high precision of positioning of the compensator because of a capacity of a memory. The most effective positioning system for the compensator of the zoom lens is a so-called TTL or semi-TTL auto-focusing system. In an active system in which a light flux is projected and a return light is detected, it is necessary to provide both a projection light source and a photo-sensing element. As a result, a structure is complex and additional parts are required. In a system in which defocus state of an image is detected based on an image signal from an imaging device arranged on a predetermined image plane of the zoom lens, a focusing time is relatively long.